In sawing rods or blocks of semiconductor material, in particular silicon, into thin wafers typically 0.1 to 1 mm thick, use is primarily made of annular saw blades since the high requirements relating to the geometrical quality of the wafers can best be met therewith. The saw blade has a cutting edge which is usually a matrix of metal such as, for example, nickel in which grains of a hard material such as, for instance, diamond are embedded and which affect the actual removal of material, which matrix surrounds the annular hole. However, even with such annular saw blades, the saw blade exhibits deviations from the desired line of cut on its path through the workpiece in the course of the sawing operation. Consequently, some of the wafers obtained exhibit geometrical faults such as thickness variation, and warp or bow to a greater or lesser degree. With ever increasing requirements being imposed by the manufacturers of electronic components on the wafer geometry, this results in an increasing number of wafers which are excluded from further processing owing to geometrical defects.
In order to avoid such losses, use is being made to an increased extent of processes in which this deviation of the saw blade from the desired specified line of cut during the sawing operation can be smoothed out as, for example, by exerting regulating forces on the saw blade. However, a prerequisite for this is that high performance processes are available which make it possible to monitor the path of cut with high accuracy during every sawing operation.
At present, eddy-current sensors are mainly used for this purpose. The basic principle of measurement is that the sensor induces, by means of a coil with a high-frequency alternating current flowing through it, eddy-currents in the saw blade of the annular saw which in turn produces, as a function of the sensor/saw blade distance, changes in impedance from which a voltage change proportional to this distance can ultimately be derived. Normally one or more sensors are installed in a fixed position relative to the saw blade and register the changes in distance from the saw blade during sawing which can be converted into regulating pulses for smoothing out the deviations, possibly via a control unit.
A disadvantage in the case of these monitoring methods using conventional eddy-current sensors is their susceptibility to malfunction when exposed to the cooling lubricant, in most cases water containing surfactants, which are indispensable when sawing with an annular saw blade. In the sawing operation, this is injected into the sawed slit and during this process is entrained in varying amounts by the saw blade, and is also distributed more or less heavily over the side faces thereof, which may result in an appreciable error in the distance values measured. This troublesome effect is particularly crucial in those positions which are actually best suited for the monitoring namely, when it is intended to track the path of the cut through the wafer being produced. In this case, reliable measured values are no longer obtained with the conventional sensors. This is particularly disadvantageous if regulation or correction measures are undertaken at the saw blade or the workpiece on the basis of these values which may result in serious errors and malfunctions, especially in the case of automated processes.